This invention is a defrost control system for a refrigerator that automatically varies the length of time between defrosts in response to the rate of frost accumulation.
The accumulation of build up of frost on the evaporator of a refrigerator requires that a defrosting system be employed. Various automatic defrosting systems have been employed and are well known in the art. Typically, an automatic defrost system is controlled by an electro-mechanical timer which initiates operation of the defrost system at fixed intervals of clock time, or after the compressor has run a predetermined length of time. The rate at which frost forms on the evaporator is a function of the amount of water vapor in the air passing over the evaporator, the greater the water content the faster the frost accumulates. In a refrigerator, the amount of water vapor within the air to be cooled depends a great deal on the ambient conditions (i.e., room temperature and relative humidity) outside the refrigerator because ambient air is introduced into the refrigerator each time the door is opened and closed, and water vapor sources (e.g. wet produce and open containers of liquid) within the refrigerator. With defrost systems controlled only with respect to time and with a slow build up of frost, operation of the defrost system is sometimes initiated before any significant amount of frost has built up on the evaporator, thus resulting in a wastage of power to defrost the refrigerator when it is not required and exposing the items in the refrigerator to unnecessary defrost cycles.
Another defrosting system is one in which the number of door openings are counted and a defrosting cycle is initiated after a selected number of openings occur. This arrangement is disadvantageous in that an unused or little used refrigerator would not be defrosted even though a substantial frost deposit has built up.